MP6: Navigation menu is not sticky when in the Jetpack menu

Description

On any page of your dashboard, scroll down: the navigation menu sticks and remains visible as you scroll

Open the Jetpack menu, and scroll down: the navigation menu is not sticky anymore.

We added this rule in r729476, to color the arrow in green in order to match the cloud background in the Jetpack menu. Obviously, this is far from ideal as it breaks the consistency with the rest of the dashboard. We should make sure that Jetpack doesn't break the general rules added by MP6.